The production of high-purity hydrogen is of great interest industrially, the latter being widely used in many synthetic processes, such as hydrocracking, the production of methanol, the production of oxoalcohols and isomerization processes.
In the prior art, PSA processes have proved to be very efficient in the separation of gas mixtures and in particular in the production of pure hydrogen or oxygen from gas mixtures contaminated by various impurities. PSA processes take advantage of the adsorption selectivity of a given adsorbent for one or a number of the contaminating substances of the gas mixture to be purified.
The choice of the adsorbent is problematic: it depends, on the one hand, on the nature of the mixture to be treated. As a general rule, the adsorbents are selected according to their ability to adsorb and to desorb a specific compound. In fact, PSA processes involve the operation of pressure cycles. In a first phase, the adsorbent bed separates at least one constituent of the mixture by adsorption of this constituent on the adsorbent bed. In a second phase, the adsorbent is regenerated by lowering the pressure. At each new cycle, it is therefore essential for the desorption to be efficient and complete, so that there is an identical regenerated state at each new cycle. However, it is clear that this ability to adsorb and then desorb a specific constituent of a gas mixture is a function of the specific operating conditions of the PSA process envisaged and in particular of the temperature and pressure conditions.
A distinction must therefore be made between PSA processes intended for the production of oxygen, which generally operate at adsorption pressures of less than 5.times.10.sup.5 Pa, and PSA processes intended for the production of hydrogen, which can involve adsorption pressures of between 5.times.10.sup.5 and 70.times.10.sup.5 Pa.
However, insofar as the mixture to be purified generally comprises more than one impurity, it is desirable for the adsorbent to be able to adsorb and then desorb not one alone but a number of these impurities.
In point of fact, the adsorption profile and selectivity for a given constituent are often influenced by the presence, in the gas mixture, of other impurities, this being due, for example, to possible competition or to poisoning of the adsorbent.
These various considerations account for the complexity of the problem of the optimization of PSA processes by improvement of the adsorbent.
Recent studies have shown that, in the case of mixtures containing nitrogen, oxygen, hydrogen, methane and argon or helium, lithium-exchanged zeolites make possible a marked improvement in the performance characteristics. The result in particular of the various research studies carried out is that the selection criteria to be taken into account in choosing the adsorbent are its nitrogen adsorption capacity, its nitrogen/oxygen selectivity, its mechanical strength (the packing of the adsorbent having to be possible over a certain height, without crushing) and the pressure drop occasioned, this naturally being the situation in the case of gas mixtures comprising both nitrogen and oxygen for the purpose of purification of the oxygen.
Reference will be made, for example, to documents U.S. Pat. No. 5,152,813 and U.S. Pat. No. 5,258,058 and to Patent Application EP-A-0,297,542, which describe the use of lithium-exchanged zeolites of type X in PSA processes intended for the production of oxygen.
The teaching of these documents, however, is not generally applicable to the purification of gas mixtures containing impurities of the carbon monoxide, carbon dioxide or C.sub.1 -C.sub.8 hydrocarbon type, the presence of which modifies the profile for adsorption of nitrogen by the zeolite. In point of fact, these impurities are the most frequently encountered in PSA units for the purification of hydrogen.
Moreover, the adsorption pressures involved in the prior art cited, generally being well below 5.times.10.sup.5 Pa, do not correspond to those generally used for PSA processes for the production of hydrogen.
Indeed, as regards the production of hydrogen from a hydrogen-based gas mixture containing CO, CO.sub.2, CH.sub.4, NH.sub.3, H.sub.2 S, N.sub.2 and H.sub.2 O as impurities, document U.S. Pat No. 3,430,418 provides the combination of two types of adsorbent, the first, which is an active charcoal, removing CH.sub.4, CO.sub.2 and H.sub.2 O and the second, which is a zeolite of type A containing calcium, making possible the removal of the nitrogen and carbon monoxide. Until now, so as to improve the performance characteristics of PSA processes for the production of hydrogen, and in particular with a view to obtaining a better hydrogen yield, the number and the arrangement of the adsorbent beds operating in parallel has essentially been varied. Documents U.S. Pat. No. 4,381,189 and FR-A-2,330,433 illustrate in particular such an approach.